Passive intruder detection systems are widely employed to detect the presence and movement of an intruder in a protected region. In the typical case, optics, operatively associated with an infrared detector, provide one or more fields of view which image infrared energy onto the active sensing element of the detector. The detector is operative in response to the thus received infrared energy to provide a signal indication of a possible intruder.
The confidence level of the security system critically depends on the ability to reliably distinguish true intruder events from false alarm producing events in the operative locale of the sensor. Thermal activity in the fields of view of the infrared detector is particularly troublesome, as space heaters, animals, and other warm objects induce false alarms as well as air convection, sunlight with cloud motion, and other kinds of thermal instabilities.
Dual element balanced detectors, for example as disclosed in U.S. Pat. Nos. 4,364,030, 3,839,640, 4,343,987, 4,514,631, and 4,707,604, each incorporated herein by reference, provide "common mode" rejection of randomly varying thermal noise. These detectors have dual elements that produce opposite polarity electrical signals when exposed to thermal activity. The signals are combined, and randomly varying signals are self-cancelling over time.
Detectors based on the principle of common mode thermal noise rejection are subject to degraded performance to the extent that one or the other element of the dual element balanced detectors is viewing a dissimilar background from the other element. The elements exposed to dissimilar backgrounds are effectively prevented from producing self-cancelling signals, whereby the detectors are subjected to false alarms. Typically, the fields of view are subject to splitting into dissimilar backgrounds by furniture or a wall in the surveillance zone. While installers are usually cautioned to avoid placing the detectors in positions where any one or more of their associated fields of view could become split, in point of fact for many installations it is often difficult or impossible to do so.